DERBY-6169: Reduce visibility of classes and methods under impl/sql

Changes under impl/sql/compile:

- Reduce visibility of some classes, fields and methods

- Remove some unused methods (including one actually used setter
  method where the field it sets is unused)

- Make last parameter of UnaryOperatorNode.init() a scalar rather
  than an array, since it's always a single value

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1468317 13f79535-47bb-0310-9956-ffa450edef68